The present invention relates to cementitious compositions and their uses.
In particular, this invention concerns water resistant cementitious compositions comprising calcium sulphate hemihydrate and portland cement, and is particularly concerned with cementitious compositions for use as exterior mortars, rendering compositions and cast articles and the like, and in applications in which water resistance, good surface finish and a rapid gain in strength in the early stages following application are important.
Both calcium sulphate hemihydrate and portland cement can individually be reacted with water and used in the applications outlined above, but there are both advantages and disadvantages associated with each of these cementitious materials. Calcium sulphate hemihydrate, when hydrated with water, possesses good workability before setting commences, and also shows a useful gain in strength within the first two hours of application. It can also be applied to give a fine surface finish. It is, however, appreciably soluble in water, and therefore unsuitable for use where it will be exposed to weathering. Portland cement, when hydrated with water, gains strength only slowly, shows a poorer surface finish than calcium sulphate-based compositions, but is relatively insoluble in water.
A product formed by hydrating a mixture of calcium sulphate hemihydrate and portland cement might be expected to give the advantages of each of these two cementitious materials, while diluting the effects of their individual weaknesses. Unfortunately, however, chemical reactions occur between sulphate ions, which are supplied principally by the calcium sulphate, and aluminium compounds in the hydrated portland cement. For example tricalcium aluminate and hydrated calcium aluminosulphate (CaO).sub.3.Al.sub.2 O.sub.3.CaSO.sub.4.(H.sub.2 O).sub.12 !, produce a hydrated calcium aluminosulphate of large crystal volume which may be represented by the formula (CaO).sub.3.Al.sub.2 O.sub.3.(CaSO.sub.4).sub.3.(H.sub.2 O).sub.32, and which will be referred to hereinafter as "Aft". The expansive forces introduced into a hardened cementitious product by the formation of Aft can cause cracking and subsequent terminal deterioration of the product.